build_first_coarse_grid_candidate.h

Go to the documentation of this file.

/*
 * Copyright 2023 MusicScience37 (Kenta Kabashima)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once
 
#include <limits>
#include <optional>
 
#include "num_collect/base/index_type.h"
#include "num_collect/linear/impl/amg/node_connection_list.h"
#include "num_collect/linear/impl/amg/node_layer.h"
#include "num_collect/util/vector.h"
 
namespace num_collect::linear::impl::amg {
 
template <typename StorageIndex>
[[nodiscard]] inline auto compute_node_scores(
    const node_connection_list<StorageIndex>& transposed_connections)
    -> util::vector<StorageIndex> {
    util::vector<StorageIndex> scores(transposed_connections.num_nodes());
    for (StorageIndex i = 0; i < transposed_connections.num_nodes(); ++i) {
        scores[i] = static_cast<StorageIndex>(
            transposed_connections.connected_nodes_to(i).size());
    }
    return scores;
}
 
template <typename StorageIndex>
[[nodiscard]] auto find_max_score_index(
    const util::vector<StorageIndex>& scores,
    const util::vector<node_layer>& classification)
    -> std::optional<index_type> {
    std::optional<index_type> index;
    auto score = std::numeric_limits<StorageIndex>::min();
    for (index_type i = 0; i < scores.size(); ++i) {
        if (classification[i] == node_layer::unclassified) {
            if (scores[i] > score) {
                index = i;
                score = scores[i];
            }
        }
    }
    return index;
}
 
template <typename StorageIndex>
[[nodiscard]] inline auto build_first_coarse_grid_candidate(
    const node_connection_list<StorageIndex>& connections,
    const node_connection_list<StorageIndex>& transposed_connections)
    -> util::vector<node_layer> {
    util::vector<node_layer> classification(
        connections.num_nodes(), node_layer::unclassified);
 
    util::vector<StorageIndex> scores =
        compute_node_scores(transposed_connections);
 
    while (true) {
        auto selection = find_max_score_index(scores, classification);
        if (!selection) {
            break;
        }
 
        classification[*selection] = node_layer::coarse;
        for (const auto j :
            transposed_connections.connected_nodes_to(*selection)) {
            if (classification[j] == node_layer::unclassified) {
                classification[j] = node_layer::fine;
                for (const auto k : connections.connected_nodes_to(j)) {
                    scores[k] += 1;
                }
            }
        }
        for (const auto j : connections.connected_nodes_to(*selection)) {
            if (classification[j] == node_layer::unclassified) {
                scores[j] -= 1;
            }
        }
    }
 
    return classification;
}
 
}  // namespace num_collect::linear::impl::amg